Apparatus for determining the drawing quality of sheet steels



Aug. 28, 1962 E. s. MADRZYK ETAL 3,050,991

APPARATUS FOR DETERMINING THE DRAWING QUALITY OF SHEET STEELS FiledMarch 2, 1959 2 Sheets-Sheet 1 W fierzczlii, 62360 Aug. 28, 1962 E. s.MADRZYK ETAL 3,050,99

APPARATUS FOR DETERMINING THE DRAWING QUALITY OF SHEET STEELS FiledMarch 2, 1959 2 Sheets-Sheet 2 3,659,991 APPARATUS FOR DETEPMENH IG THEDRAWHNG QUALITY OF MEET STEELS Edmund S. Madrzylr and Francis E. Gibson,Lansing,

Ill., assignors to inland Steel Qornpany, a corporation of DelawareFiled Mar. 2, 195?, fies. No. 7%,407 6 Claims. (Qt. 73-87) Thisinvention relates to apparatus for quantitatively evaluating the formingcharacteristics of metallic materials of construction. It is morespecifically concerned with physically determining the drawing qualityof sheet metals.

Although the forming of sheet metal structures by drawing operations isone of the most common sheet metal fabricating operations, nosatisfactory method for evaluating the drawability of metal sheet hasbeen developed. The drawability of a metal is rather an obscure propertyand much effort has been expended to find a method that would adequatelymeasure this property. There have been many tests proposed and devisedto obtain an indication of the probable behavior of a sheet under thepress. These include simple bend tests, slottedstrip tests,tear-strength tests, mechanical and hydraulic O cupping tests, andtensile tests. None of these tests, however, seem to provide an accurateindex of drawability when compared to actual press performance.

Another type of formability test which has been investi gated involvesthe forming of a bulge on a suitably sized test specimen by means ofhydraulic pressure with the sheet metal specimen clamped over a circularor oval opening. The stress-strain relationships involved in a hydraulicbulge test have been mathematically analyzed and expressions thatconform very closely to observed data have been derived. Theinterpretation of bulge data and the application of the results of suchhydraulic bulge testing to commercial operations, however, have not beenclarified.

According to this invention it has been found that the instability pointof a metal under biaxial tension (ap plied by means of a hydraulic bulgetester) can be used as a sensitive criterion of drawability. Theinstability point is defined as the point at which plastic fiow proceedswithout increases in pressure. It has been found that, in bulge testing,the upper portion of the bulge curve, when pressure is plotted againstheight of bulge, is parabolic in nature which allows the preciselocation of the instability point. This point so determined has beentermed the H value. A drawability index based on this value shows moreability to discriminate between small variations in drawing quality thanany of the common tests heretofore employed.

FIGURE 1 is a plan View of one embodiment of the bulge tester of thisinvention;

FIGURE 2 is a side elevation View of the apparatus shown in FIGURE 1;

'FIGURE 3 is an enlarged fragmentary view illustrating the specimenclamping means employed in the illustrative embodiment; and

FIGURE 4 is an enlarged fragmentary view illustrating the passagewaythrough which the hydraulic operating fluid is admitted to the internalchamber.

In the drawings is illustrated a specific embodiment of an apparatuswhich can be used for carrying out the bulge testing of a sheet metalspecimen to evaluate its drawability qualities. The apparatus in thedrawings comprises a heavy duty base ltl having internal chamber 11 witha circular opening. Upstanding wall 12. of chamber 11 is penetrated by aplurality of ports 13 and 14 which connect respectively by means ofpassageways 13 and 14' (not shown) with hydraulic fluid inlet 15 andoutlet 16 means. Suitable flow controllers 17 and 18 are provided toregulate the flow of hydraulic fluid through the system. Base It hasWall thicknesses suificient to withstand safely pressures of 5,000-10000 p.s.i. depending upon the service in which the bulge tester will beemployed.

A portion of the upper surface of sidewall 12 is undercut to providerecessed shoulder 19 which is surrounded by the remaining portion 24 ofthe upper surface of sidewall 12. A plurality of holes 21 traversesidewall 12 to receive the holddown clamp fasteners 22 which cooperatewith an internally threaded fastener 23 disposed in recess 24 located inthe bottom surface of base 16. The sheet metal test specimen S cut inthe form of a disc is laid on the surface of recessed shoulder 19 andcovers the circular opening in internal chamber Ill. The diameter of thetest specimen 8 is sufficiently larger than the diameter of the openingto properly bear on the face of recessed shoulder 19 and provide enougharea for clamping ring 25 to engage in a non-slip relationship with thetest specimen.

In the illustrated embodiment clamping ring 25 is in the form of anannulus having an inner diameter substantially equal to the diameter ofthe chamber opening. A portion of the lower surface of clamping ring 25is undercut. The remaining portion of the lower surface of clamping ring25" engaging the test specimen is provided with an anti-skid surfaceconsisting of a plurality of lands and groves. The cooperating portionof the face of recessed shoulder 19 is similarly surfaced. Thecooperating non-skid surfaces of base member lit and clamping ring 25hold the test specimen securely in place and prevent slippage andleakage when an adequate clamping means is provided.

A satisfactory clamping arrangement is shown in the drawings andconsists of a plurality of segmental clamps 2 6 which contact clampingring 25 throughout the complete circumference. Ten segments are employedin the illustrated example which permit the use of two fasteners 22 perclamp. It will be noted that the outer marginal edge of clamp 26 restson the remaining portion 24 of the upper surface of the sidewall face.The inner marginal edge rests on the upper surface of clamping ring 25.When fasteners 22 are drawn up tightly there is a slight bowing of thesegmental clamps 26 which is compensated for by the under cut portion ofthe lower surface of clamping ring 25- and permits the application of amore effective clamping action than is provided by an unbroken surface.

A suitable indicating and measuring device 30 such as an extensometer issuspended over the test specimen. S and the sensing element 3b" broughtinto contact with the center of the specimen. To insure properpositioning of extensometer 31? at the pole of the test specimen atripod jig 31 is employed. Jig 31 has three legs 32 having verticallyadjustable, pointed feet 33 which register with suitable bench markslocated in plate 34 upon which the base member It rests. Legs 32 areconnected by spider 35 upon which extensometer 30 is mounted.

Pressure indicating gauges 36 are connected to output means 16 by pipingmanifold 37. The selection of gauges will depend upon the test programbeing carried out. Pump means (not shown) is connected to inlet means 15which forces a suitable hydraulic fluid into chamber 11 to develop thefluid pressure required to bulge the test specimen.

For safety reasons it is preferred that covered shield 4t be used toprevent the spraying of the hydraulic fluid in the event that anunexpected rupture in the test specimen occurs.

Employing the apparatus of this invention a simple testing procedure isemployed to determine the drawability index of the sheet metal beingtested. After the sheet metal test specimen is securely clamped in place(with the illustrated apparatus a pneumatic impact wrench is used to runin the threaded fasteners and secure the segmental clamps) a hydraulicfiuid is pumped into chamber 11 under sufficient pressure to effectbulging of the test specimen and the bulge height and pressuremeasurements are correlated and recorded. Pressure is increased until asufiicient bulge height is produced to permit the determination of thedrawability index. This condition is indicated when bulge heightincreases continue without further increase in pressure.

By employing these simple measurements the biaxial instability pointwhich is the onset of failure is accurately located. This point islocated by plotting increments of pressure against increments of bulgeheight and obtaining a curve, the upper terminal portion of which has aparabolic configuration represented by the mathematical expressionWhere:

p pressure in pounds per square inch, h=bulge height in inches, and C CC =constants characteristic of metal tested.

In accordance with this invention only this portion of the bulge curveis used for evaluating the drawability of the sheet metal beinginvestigated.

By differentiation, the above formula becomes The maximum was obtainedby setting this equal to zero:

The value of the maximum read on the h axis was found by solving for H,where H is the drawability index.

In practice, in order to evaluate the constants, three bulge points areused within the parabolic range; one being near the highest part of thecurve, another being at the beginning of the parabolic section of thebulge curve, and still another being intermediate the first and secondpoints.

The correesponding pressure values are read from the actual bulge dataand these three sets of points substituted into three parabola equationshaving three unknown constants, C C and C The H value is then determinedby the suitable computation.

It is apparent in determining the H value that the gauge of the metalspecimen which is being investigated has an effect. In order tocorrelate bulge data it is preferred that standard gauge be selected asthe norm and compensation be made for any variations from this gauge bythe samples which are tested. In establishing an illustrative procedurefor employing the instant invention a gauge thickness which approximatesthe thickness of steel sheet metal employed in a wide variety of drawingoperations was selected, namely, 0.050. In setting up the measuringinstrument this thickness is established as the standard and anydeviation from this gauge, either above or below, is taken intoconsideration in the calculations for determining the H value byappropriate corrections. In applying this correction it has been foundfor sheet steel specimens that a correction of 5.636 inches per inch ofgauge is to be added to the H value determination if the gauge is lessthan the standard or subtracted from the H i value if the gauge value isgreater than the standard. This correction of 5.636 inches per inch ofgauge was determined empirically by correlating the efiect of gauge on Hvalue for a number of samples varying in gauge from 0.010" to 0.060.Accordingly, if the instant invention is to be employed in evaluatingthe drawability of metals other than steel an appropriate gaugecorrection will have to be experimentally determined by investigatingthe effect of gauge of the sample metal on H value and calculating anappropriate correction which can be applied to the specific metalinvolved.

For example, in determining the drawability index of 2 killed, lowcarbon, aluminum deoxidized steel sheet 0.046 inch thick, a disc havinga diameter of about 14 inches was prepared. The bulge tester used had abase member having an inner chamber 10 inches in diameter and 1% inchesdeep. The bottom was inch thick and the sidewall was 2% inches high and6 inches thick. The upper surface of the side wall was undercut inchleaving a inch thick portion of the surface to provide a recessedshoulder enclosed by the outer surface of the sidewall. Clamping ring,/16" thick, having a 10 inch I.D., 14 /8 inch 0.1)., was used to retainthe test specimen securely during testing. To prevent slippage anon-skid surface was provided on the cooperating faces of the ring andbase. Both the base and clamping ring were fabricated from A.I.S.l. 1065steel. To hold the clamping ring in place ten segmental clamps wereused. Two holes were provided in each segmental clamp to receive 1 inchcap screws which engage with nuts mounted in recesses in the base. Afterthe test specimen was put in place and the apparatus assembled the capscrews were driven to a tightened condition by a pneumatic Wrench.Hydraulic pressure was applied to the test specimen by pumping ahydraulic oil into the inner chamber.

Pressure (p) versus height (11) readings were taken every 10 poundsincremental of pressure change and the following bulge data obtained:

BULGE DATA The data were graphically correlated by plotting p as theordinate v. h as the abscissa.

Three sets of points were chosen from the parabolic portion of the bulgecurve, viz.

=7s3.0 h=3.0 p=707.5 h=2.6 p=625.0 h=2.2

which were substituted in the general parabolic formula. To each of the11 readings is added the difference in gauge between the standard of0.050 and the gauge of the sample, i.e., 0.046".

This was solved for H value by a determinant The empirical gaugecorrection was determined 5.636 inches/inch of gauge .004=0.02

and the corrected H value established STANDARD S Breakage for Min. U H!I percent Part Minimun It H7 1 Plenum Side Panel-Automobile bodyInstrument Panel-Automobile body Rear Quarter Side Outer-Automobile bodyBlower Housing Trailer Bathtub Refrigerator Door Liner OilPan-Automobile engine Rear Quarter Outer-Automobile body DoorOuter-Automobile body It has been found that the best performing steelshad the highest bulge value in comparable performance tests. From thesetests it was concluded that (1) The drawability index is sensitiveenough to discriminate between small difierences in drawing quality.

(2) Drawability index test values are completely reliable.

(3) The Olsen and Rockwell tests are not sensitive enough todiscriminate between small or moderate differences in drawing quality.

(4) Drawability index standards for specific parts can be convenientlyestablished for control purposes.

Although the foregoing description of the instant invention is concernedwith the testing of steel, other drawable metals such as aluminum,brass, titanium, copper, and others can also be evaluated using theprocess of this invention. To provide accurate data for steel specimensthe diameter of the effective area of the test speciment which is to bebulged should be 6 to 12 inches for example, using gauges of 0.01 to0.10 inch for example. The size of the specimen is selected to permitthe use of convenient pressures and bulge heights. Accordingly, otherdiameters and gauges can be used for other materials if desired orneeded.

In order to facilitate carrying out the determination of the drawabilityindex other apparatus embodying the characteristics of the illustrateddevice can be used. It is especially helpful if quick acting clamps ofsuitable elfectiveness be used for clamping the clamping ring whichholds the specimen in place. The test can also be made more routine byutilizing conventional analog or digital computers for determining thevalues of the constants C in the parabolic equation for the selectedparabolic portion of a specific bulge curve. For a given set ofconditions for the metal specimen, viz, gauge and diameter, it has beenfound that the parabolic portion of the curve vegin at a substantiallyconstant height. For example, the parabolic section of the curve for 10inch diameter sheet steel specimens having a gauge in the vicinity of0.050" begins at a bulge height of about 1.8 inches. Accordingly, by theuse of standard test conditions and employing appropriate pressure andheight measuring instruments suitable electrical signals can begenerated at the three preselected points on the parabolic portion ofthe curve and the output applied to a suitable automatic computer. Thiswould permit the desired value to be automatically and rapidly obtainedand would increase the effectiveness of the invention as a qualitycontrol device. In installations employing computers a suitableelectronic digital computer which can be employed is the Alwac Model IIIcomputer maunfactured and sold by the Alwac Division of El-Tronics,Inc., Redondo Beach, California.

It is apparent from the foregoing discussion that a numer of variationsand modifications can be made by those skilled in this art Withoutdeparting from the scope of this invention.

What is claimed is:

1. In an apparatus for determining the drawability in dex of a specimensheet metal disk, a rigid base member being provided With an open-face,internal cavity, an upstanding wall on said base member surrounding saidcavity, said Wall being of sufficient thickness to withstand hydraulicpressures encounter in determining said drawability index withoutdistortion, said wall having a recessed portion in the upper facethereof contiguous with the edge of said wall surrounding said cavity,said recessed portion having a non-skid marginal portion adjacent theaforesaid edge, a specimen locking ring comprising a plate adapted to bedisposed within said recessed portion and having a curved inner openingwith a peripheral configuration similar to the peripheral outline ofsaid cavity, the bottom of said plate having a non-skid marginal portionadjacent the edge of the inner opening cooperating with said non-skidsurface on said recessed portion to securely hold a sheet metal specimenclamped therebetween, the thickness of the non-skid marginal portion ofsaid ring being greater than the thickness of the remaining portion ofsaid ring, clamping means resting on said upstanding Wall and saidspecimen locking ring for securely clamping said ring in said recessedportion, means for applying a clamping force to said clamping means, andmeans for introducing a hydraulic fluid into said cavity.

2. In an apparatus for determining the drawability index of a specimensheet metal disk, a rigid base member being provided with an open-face,centrally disposed, circular internal cavity, an upstanding Wall on saidbase member surrounding said cavity, said wall being of sufficientthickness to Withstand hydraulic pressures encountered in determiningsaid drawability index without distortion, said Wall having a recessedportion in the upper face thereof contiguous with the edge of said wallsurrounding said cavity, said recessed portion having a nonskid marginalportion adjacent the aforesaid edge, a specimen locking ring comprisinga plate adapted to be disposed within said recessed portion and having acircular opening with a diameter not greater than the diameter of saidcavity, the bottom of said plate having a non-skid marginal portionadjacent the edge of the inner opening cooperating with said non-skidsurface on said recessed portion to securely hold a sheet metal specimenclamped therebetween, the thickness of the non-skid marginal portion ofsaid ring being greater than the thickness of the remaining portion ofsaid ring, clamping means resting on said upstanding wall and saidspecimen locking ring for securely clamping said ring in said recessedportion, means for applying a clamping force to said clamping means, andmeans for introducing a hydraulic fluid into said cavity.

3. In an apparatus for determining the drawability index of a specimensheet metal disk, a rigid base member being provided with an open-face,centrally disposed, circular internal cavity, an upstanding wall on saidbase member surrounding said cavity, said wall being of sufficientthickness to Withstand hydraulic pressures encountered in determiningsaid drawability index without distortion, said wall having a recessedcircular portion in the upper face thereof contiguous with the edge ofsaid Wall surrounding said cavity, said recessed portion having a nonskid marginal portion adjacent the aforesaid edge, an annular, specimenlocking ring comprising a plate adapted to be disposed within saidrecessed portion and having a circular opening with a diameter notgreater than the diameter of said cavity, the bottom of said platehaving a non-skid marginal portion adjacent the edge of the inneropening cooperating with said non-skid surface on said recessed portionto securely hold a sheet metal specimen clamped therebetween, thethickness of the non-skid marginal portion of said ring being greaterthan the thickness of the remaining portion of said ring, clamping meansresting on said upstanding wall and said specimen locking ring forsecurely clamping said ring in said recessed portion, means for applyinga clamping force to said clamping means, and means for introducing a hydraulic fluid into said cavity.

4. In an apparatus for determining the drawability index of a specimensheet metal disk, a rigid base member being provided with an open-face,centrally disposed, circular internal cavity, an upstanding wall on saidbase member surrounding said cavity, said wall being of suflicientthickness to withstand hydraulic pressures encountered in determiningsaid drawability index Without distortion, said wall having a recessedcircular portion in the upper face thereof contiguous with the edge ofsaid wall surrounding said cavity, said recessed portion having anon-skid marginal portion adjacent the aforesaid edge, an annular,specimen locking ring comprising a plate adapted to be disposed withinsaid recessed portion and having an outer diameter substantially lessthan the diameter of said recessed portion, and having a circularopening with a diameter not less than the diameter of said cavity, thebottom of said plate having a non-skid marginal portion adjacent theedge of the inner opening cooperating with said non-skid surface on saidrecessed portion to securely hold a sheet metal specimen clampedtherebetween, the thickness of the non-skid marginal portion of saidring being greater than the thickness of the remaining portion of saidring, clamping means resting on said upstanding wall and said specimenlocking ring for securely clamping said ring in said recessed portion,means for applying a clamping force to said clamping means, and meansfor introducing a hydraulic fluid into said cavity.

5. In an apparatus for determining the drawability index of a specimensheet metal disk, a rigid base member being provided with an open-face,centrally disposed, circular internal cavity, an upstanding wall on saidbase member surrounding said cavity, said wall being of sufiicientthickness to withstand hydraulic pressures encountered in determiningsaid drawability index without distortion, said wall having a recessedcircular portion in the upper face thereof contiguous with the edge ofsaid wall surrounding said cavity, said recessed portion having anon-skid marginal portion adjacent the aforesaid edge, an annular,specimen locking ring comprising a plate adapted to be disposed withinsaid recessed portion and having an outer diameter substantially lessthan the diameter of said recessed portion, and having a circularopening with a diameter substantially equal to the diameter of saidcavity, the bottom of said plate having a non-skid marginal portionadjacent the edge of the inner opening cooperating with said non-skidsurface on said recessed portion to securely hold a sheet metal specimenclamped therebetween, the thickness of the non-skid marginal portion ofsaid ring being greater than the thickness of the remaining portion ofsaid ring, clamping means resting on said upstanding wall and saidspecimen locking ring for securely clamping said ring in said recessedportion, comprising a plurality of segmental plates forming an annularring congruent with the face of said upstanding Wall, means for applyinga clamping force to said clamping means, comprising threaded fastenerssecuring said segmental plates to said wall and means for introducing ahydraulic fluid into said cavity.

6. An apparatus for determining the drawability index of a specimensheet metal disk which comprises a rigid base member being provided withan open-face internal cavity, an upstanding wall on said base membersurrounding said cavity, said wall being of sufficient thickness towithstand hydraulic pressures encountered in determining saiddrawability index without distortion, said wall having a recessedportion in the upper face thereof contiguous with the edge of said wallsurrounding said cavity, said recessed portion having a non-skidmarginal portion adjacent the aforesaid edge, a specimen locking ringcomprising a plate adapted to be disposed within said recessed portionand having a curved inner opening with a peripheral configurationsimilar to the peripheral outline of said cavity, the bottom of saidplate having a nonskid marginal portion adjacent the edge of the inneropening cooperating with said non-skid surface on said recessed portionto securely hold a sheet metal specimen clamped therebetween, thethickness of the non-skid marginal portion of said ring being greaterthan the thickness of the remaining portion of said ring, clamping meansresting on said upstanding Wall and said specimen locking ring forsecurely clamping said ring in said recessed portion, means for applyinga clamping force to said clamping means, means for introducing ahydraulic fluid into said cavity, means for sensing the deformation ofsaid clamped metal specimen, means for indicating said deformation, andmeans for measuring the pressure of said hydraulic fluid.

